1. Field of the Invention
The present invention concerns device for generation of an exposure plan, a method for generation of an exposure plan as well as a computer program product.
2. Description of the Prior Art
One possible therapy option for many tumor types (for example for malignant tumors of the prostate, colon, breast, thyroid or central nervous system) is radiation therapy. Tumor tissue is irradiated with ionizing high-energy rays (predominantly high-energy gamma radiation or x-ray radiation, but also electrons, neutrons, protons). For all cited radiation types, the effect of the irradiation is physically based for the most part on the energy transfer in scatter processes that leads to a destruction of tumor tissue. The fact is utilized that tumor tissue is for the most part more radiation-sensitive than the surrounding normal tissue. The therapeutic effect requires high doses, typically of 20 to 100 gray dependent on the tumor type.
In order to keep the side effects low, the exposure is often divided into a number of daily individual doses (fractioning) and is administered over multiple weeks (protraction). Moreover, the exposure is spatially and energetically set such that the radiation predominantly strikes only the malignant, pathological region.
For this purpose, an exposure plan is typically generated using an image of the patient that was generated with a three-dimensional imaging method. Computed tomography images (CT images) are typically used for this. The target volume of the exposure can be established using the CT images and a surrounding tissue to be protected (for example neuronal tissue) can be identified.
Moreover, the intensity values of the image voxels of a CT image (measured in units known as “Hounsfield units”) reproduce in good approximation the electron density at the corresponding location in the body of the patient, since the intensity values of the image voxels are based on an absorption of the x-ray radiation at the associated locations. Since, in a therapeutic exposure, the intensity of the interaction of the radiation correlates with the electron density in the body, the attenuation of the radiation upon passage through the body can be calculated relatively simply from a CT image. Due to this property, CT images conventionally have been the preferable type of image for use in the generation of an exposure plan.
Recently, however, increasingly more precise exposure methods (modalities) have been developed, such that the delivery of a majority of the energy of the radiation can be limited to a focus a few millimeters in size and can even be modulated within the focus in the framework of therapy is known as intensity-modulated radiation therapy. The soft tissue contrast of a CT image increasingly is not able to match this possible precision of a therapeutic exposure. A need therefore exists to use, in the exposure plan, other imaging methods that exhibit a better soft tissue contrast.
One possible imaging method that satisfies the requirement of a better soft tissue contrast is magnetic resonance imaging (MR imaging). In such imaging, the contrast depends on the distribution of the spin densities, the interaction of the spins among one another and/or with their environment. A soft tissue contrast can be achieved that lies well above the contrast achievable with a computed tomography system. In the generation of an exposure plan, however, magnetic resonance images exhibit the significant disadvantage that the intensity values of the individual image voxels do not correlate with the electron density at the associated locations, such that the attenuation of the radiation on the path through the body cannot be sufficiently precisely determined from a magnetic resonance image.
One possibility to solve this problem is to acquire both a CT image and an MR image of a patient for an exposure plan. These two images can be registered with one another so that the image information of both images can be set in relation to one another. Due to the good soft tissue contrast, the target volume to be irradiated can be precisely localized from the MR image, and the attenuation of the radiation on the path through the body toward the target volume can be precisely determined from the CT image. The necessity to operate with two different imaging modalities in parallel, however, is disadvantageous. In addition to an increased radiation exposure for the patient, the parallel employment of the two imaging modalities means a substantially increased time and cost expenditure in the generation of the exposure plan. An adaptation of the exposure plan is often necessary, primarily when a fractioned exposure is implemented. In this case CT and MR images of the patient must be repeatedly acquired, and the disadvantages of this method become even more manifest.
Furthermore, methods are known in which anatomical structures in MR images are associated with attenuation coefficients. These methods are based on the assumption that deviations of attenuation coefficients for the specific structures (for example bones) from patient-to-patient are negligible. It is additionally relatively complicated to localize the various tissue types in an MR image—for example via interactive and/or partially automated segmentation methods—and to respectively associate the matching attenuation coefficients with the tissue types. In order to limit the effort associated with this, in practice one is limited to only three tissue types: air, bone and soft tissues. This limitation does not always allow the desired precision in the exposure plan.